Gloucester centre Twelvetrees powered over for a second-half touchdown as England ran in four tries and fly-half Owen Farrell kicked 18 points in the 38-18 RBS 6 Nations victory against Scotland. Now Lancaster must decide whether to stick with Twelvetrees for the Ireland game, when star centre Manu Tuilagi is expected to have recovered from injury.
"It is a big step up for any player to make his debut, particularly at Twickenham," said Lancaster. "We are delighted with the way he (Twelvetrees) took it to get the try but his confidence and composure has been good all week. It shows he is ready to make the step."
He added: "This time last year we had seven players making their debuts, and with Billy making his we have a young side. We are bringing on 20-odd-year-olds and it is great for us as a squad.
"There are a few selection debates to be honest. Dylan Hartley and Courtney Lawes did well when they came on. (James) Haskell was outstanding. Across the board and with Manu (Tuilagi) coming back into consideration there will be some selection decisions to make.
"We need to knuckle down to what will be a massive game against Ireland."
England's other tries came via Chris Ashton, Geoff Parling and Danny Care, while Scotland wing Sean Maitland scored on his debut and Stuart Hogg scored a late consolation touchdown for the Scots with Craig Laidlaw kicking two penalties and a conversion.
Scotland interim coach Scott Johnson admitted his side were outplayed, saying: "We were disappointing in the areas we need to work on.
"We can dream away about how we'd like to play the game of rugby but the reality is in the modern game, if you don't get the contact area right, you can dream all you like, it's fantasy, fairytales won't come true.
"We lost the battle of field position because of our inability to stop them. So the contact area, with and without the ball, was poor. Simple."